Chapter 13  Meiosis and Sexual Life Cycles
Student Guided Notes
Overview: Variations on a Theme

The transmission of traits from one generation to the next is called ________________________ or
__________________________________.

Offspring differ somewhat from their parents and siblings, which demonstrates ___________________.

Genetics is the scientific study of ________________________________________________________.
Concept 13.1 Offspring acquire genes from their parents by inheriting chromosomes.

Parents endow their offspring with coded information in the form of ____________________.

Genes are ___________________________________________________. Genetic information is
transmitted as specific sequences of the four ___________________________________ in DNA.

Most genes program cells to synthesize specific enzymes and other proteins whose cumulative action
produces an organism’s inherited traits.

In plants and animals, reproductive cells called _____________________________ transmit genes from
one generation to the next.

After fertilization (___________________________________________________________________),
genes from both parents are present in the nucleus of the fertilized egg, or ________________________.
Like begets like, more or less: a comparison of asexual and sexual reproduction.

Only organisms that reproduce ________________ can produce offspring that are exact copies of them.

In ____________________________ reproduction, a single individual is the sole parent to donate
genes to its offspring.

In ____________________________ reproduction, two parents produce offspring that have unique
combinations of genes inherited from the two parents.
Concept 13.2 Fertilization and meiosis alternate in sexual life cycles.

A life cycle is _____________________________________________________________________
_________________________________________________________________________________.
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Human cells contain sets of chromosomes.

In humans, each somatic cell has_________ chromosomes.

During mitosis, the chromosomes become condensed and are visible under a light microscope.

During mitosis, (since the chromosomes are condensed) images of the 46 human chromosomes can be
arranged in _______________________ in order of ___________________________ to produce a
______________________________________ display.
○ The two chromosomes in a pair have the same _________________________________________
________________________________________________________________________________.
○ These ________________________________________________________ pairs carry genes that
control the same inherited characters.

Two distinct _____________________________________________________, the X and the Y, are an
exception to the general pattern of homologous chromosomes in human somatic cells.
○ Human _______________________________ have a homologous pair of X chromosomes (XX);
________________________________ have one X and one Y chromosome (XY).

The other 22 pairs of chromosomes are called _____________________________________.

The occurrence of homologous pairs of chromosomes is a consequence of sexual reproduction.
○ We inherit one chromosome of each homologous pair from each parent.

The number of chromosomes in a single set is represented by ______. (ex. human n = 23)

Any cell with two sets of chromosomes is called a ________________________________ and has a
diploid number of chromosomes, abbreviated as _________.

Sperm cells or ova (gametes) in humans have only one set of chromosomes—_____________________
____________________ (in an ovum) or ____________________________________ (in a sperm cell).
○ A cell with a single chromosome set is a ________________________ cell, abbreviated as n.

In a cell in which DNA synthesis has occurred, all the chromosomes are duplicated. Each duplicated
chromosome consists of two ________________________________________________________.

It is crucial to understand the differences among homologous chromosomes, sister chromatids,
nonsister chromatids, and chromosome sets.
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The behavior of chromosome sets in the human life cycle.

The human life cycle begins when a haploid ________________________ cell fuses with a haploid
______________________ (egg), this joining of cells is called ______________________________.
○ The union of these gametes, culminating in the __________________________________________
___________________________, is fertilization. (in other words, syngamy precedes fertilization)

The fertilized egg (__________________________) is diploid because _________________________
____________________________________ bearing genes from the maternal and paternal family lines.

As a person develops from a zygote to a sexually mature adult, mitosis generates all the somatic cells of
the body.

Gametes develop from specialized cells called ____________________________ in the ____________.

Gametes undergo the process of _________________, in which the chromosome number is _________.

____________________ restores the diploid condition by combining two haploid sets of chromosomes.
Organisms display a variety of sexual life cycles.

Fertilization and meiosis alternate in all sexual life cycles, in plants, fungi, protists, and animals.

Most animals, including humans, have the FIRST TYPE OF LIFE CYCLE, in which
________________________ are the only haploid cells.
○ Gametes do not divide but fuse to form a diploid zygote that divides by mitosis to produce a
multicellular diploid organism.

Plants and some algae have a SECOND TYPE OF LIFE CYCLE called __________________________
___________________________________________________.
○ This life cycle includes two multicellular stages, one __________________________ and one
___________________________________.
○ The multicellular diploid stage is called the _____________________________________.
○ Meiosis in the sporophyte produces ________________________________________.
○ Unlike a gamete, a haploid spore doesn’t fuse with another cell but rather ___________________
_______________________________________________________________________________.
○ Gametes produced via mitosis by the gametophyte fuse to form the zygote, which grows into the
sporophyte by mitosis.
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○ In this type of life cycle, the sporophyte generation produces a ______________________________
as its offspring, and the gametophyte generation produces the next ___________________________
generation.

Most fungi and some protists have a THIRD TYPE OF LIFE CYCLE.
○ Gametes fuse to form a zygote, which is the only diploid phase.
○ The zygote undergoes meiosis to produce haploid cells.
○ These haploid cells divide by mitosis to form either unicellular daughter cells or a haploid
multicellular adult organism.
○ The haploid adult produces gametes by mitosis.
○ The only diploid stage in these species is a single-celled zygote.
SEE POWERPOINT FOR PICTURES OF THE LIFE CYCLES!

Although the three types of sexual life cycles differ in the timing of meiosis and fertilization, they share
a fundamental result: _________________________________________________________________.
Concept 13.3 Meiosis reduces the number of chromosome sets from diploid to haploid.

Many steps of meiosis resemble steps in mitosis.

Both meiosis and mitosis are preceded by _________________________________________________.

In meiosis, there are two consecutive cell divisions, meiosis I and meiosis II, resulting in _________
daughter cells.
○ The first division, meiosis I, separates _________________________________________________.
○ The second division, meiosis II, separates ______________________________________________.
○ The four daughter cells at the end of meiosis have only _____________________ as many
chromosomes as the original parent cell.

Meiosis I is preceded by interphase, in which the chromosomes are ___________________________
to form _____________________________________________.
○ Two ___________________________________________________________________ make up
one duplicated chromosome.
○ The sister chromatids are closely associated all along their length. This association is called
_______________________________________________________. (Recall this from Chapter 12!)
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
In contrast, the two chromosomes of a homologous pair are individual chromosomes that were inherited
from different parents.
○ Homologous chromosomes appear to be alike, but they may have ___________________________
_____________________________________________, each called an allele, at corresponding loci.
Crossing over and synapsis occurs during prophase I.

____________________________________________ begins very early in prophase I, as homologous
chromosomes pair loosely along their lengths.
○ Each gene on one homolog is aligned precisely with the corresponding gene on the other homolog.

In a single crossover event, the DNA of ________________________________________________ —
one maternal and one paternal chromatid of a homologous pair—is broken by specific proteins at
precisely corresponding points.
○ The two regions beyond the crossover point are joined to the other chromatid (the area where the
cross over occurs is sometimes called the ____________________________.)
○ This leads to __________________________________ among cells.
Compare mitosis and meiosis.

Meiosis halves the total number of chromosomes, reducing the number of sets of chromosomes from
two (diploid) to one (haploid), with each daughter cell receiving one set.
○ _____________________________ conserves the number of chromosome sets.

Meiosis produces cells that differ genetically from the parent cell and from each other.
○ Mitosis produces daughter cells that __________________________________________________
________________________________________________________________________________.

Three events UNIQUE TO MEIOSIS occur during meiosis I:
1. Synapsis and crossing over

During _______________________________, duplicated homologs pair up and the formation of
the ___________________________________________________________ between them
holds them in synapsis (NOTE: synapsis = formation of a tetrad).


Crossing over also occurs during this stage.
Synapsis and crossing over do not occur during prophase of mitosis.
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2. Homologs on the metaphase plate

During metaphase I of meiosis, _________________________________________________,
rather than individual chromosomes, as in mitosis, line up on the metaphase plate.
3. Separation of homologs

During anaphase I of meiosis, the _________________________________________________
_____________________________________________________________________________
_______________, while the sister chromatids of each duplicated chromosome remain attached.


In anaphase of mitosis, by contrast, the ________________________________ separate.
During meiosis I, sister chromatids are attached along their lengths by protein complexes called
cohesions.

Meiosis I is a _____________________________ division because it halves the number of chromosome
sets from two (the diploid state) to one (the haploid state).
Concept 13.4 Genetic variation produced in sexual life cycles contributes to evolution.

__________________________________ are the original source of genetic diversity.
○ These changes in an organism’s DNA create the different versions of genes known as ___________.
Sexual life cycles produce genetic variation among offspring.

Three mechanisms contribute to genetic variation arising from sexual reproduction: _____________
____________________________________________, ______________________________________,
and _______________________________________________.

Independent assortment of chromosomes contributes to genetic variability due to ________________
____________________________________________________________________________________
___________________________________________________________________________________.
○ There is a fifty-fifty chance that a particular daughter cell from meiosis I will get the maternal
chromosome of a certain homologous pair and a fifty-fifty chance that it will receive the paternal
chromosome.
○ Each homologous pair of chromosomes segregates _____________________________________
_________________________________________________________________________________
________________________________________________________________________________.
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
The number of combinations possible when chromosomes assort independently into gametes is ______,
where n is the haploid number of the organism.
○ If n = 3, there are 23 = 8 possible combinations.
○ For humans with n = 23, there are _______, or about ___________________________________
possible combinations of chromosomes.

Crossing over produces _________________________________ chromosomes, which combine genes
inherited from each parent.
○ During meiosis in humans, an average of __________________________ crossover events occur
per chromosome pair, depending on the _________________ of the chromosomes and
________________________________________________________________________________.

At metaphase II, _________________________________________ sort independently from one
another, further increasing the number of genetic types of daughter cells that are formed by meiosis.

The random nature of fertilization adds to the genetic variation arising from meiosis.
Evolutionary adaptation depends on a population’s genetic variation.

___________________________________ recognized the importance of genetic variation in evolution.

As the environment changes, the population may survive if some members can cope effectively with the
new conditions.
○ _______________________________ are the original source of different alleles, which are then
mixed and matched during meiosis.
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